The major objective of this research is to study social competence and mastery motivation in the first year of life of infants with Down syndrome and to examine the relationship between the mother-infant interaction and infant's mastery behaviors. Down syndrome infants are being studied longitudinally at 3, 6 and 8 months, and a cross-sectional sample is being studied at 12 months of age. Mastery motivation is being assessed using 12 tasks developed in this laboratory. In addition, the relationship between the infants' social and mastery behaviors and specific cognitive, and temperamental variables is being examined. Early social competence was found to be associated with developmental status at 12 months. Early social competence was also differentially associated with aspects of the mastery components and behaviors. Specifically, social abilities at 3 months were positively related to on-task transitions and negatively related to off-task transitions at 12 months. These relationships were evident for both. Effect Production and Practicing Sensorimotor Skills. The differences in the patterns of correlations between these two components lend support to the original conceptualization of mastery motivation. When compared to a sample of matched controls, the Down syndrome infants displayed no differences in the overall amounts of behavior emitted; they did, however vary in their distribution of behavior. In addition, the patterns of behavioral transitions were similar, though the Down syndrom infants tended to be more predictable in their transitions to and from looking. Finally, correlational analyses revealed that the frequency of persistence, success, and the transitions from persistence to success were highly related to developmental status for the Down infants whereas only the transition from persistence to success was related to developmental status for the non-delayed infants.